The present invention relates to a digital VTR, and in particular to a digital image reproduction apparatus and method for performing slow-motion reproduction (hereafter abbreviated to slow reproduction) by using a memory.
In a digital VTR, image signals are so recorded on magnetic tape that each field may be divided to a plurality of tracks. When slow reproduction is to be performed by using magnetic tape having image signals thus recorded thereon, image signals reproduced when the magnetic tape is traveling at low speed are written into a memory having a capacity of one field or one frame, and the image signals thus written are then read out.
The operation for writing the reproduced image signals into the memory at the time of slow reproduction will now be described. That is to say, image signals comprise a train of data in unit intervals (hereafter referred to as blocks) such as horizontal intervals. An ID signal comprising data (field ID) representing a field No. to which the block belongs and block No. is added to each block. The reproduced video signals undergo processing, such as error detection and error correction, on a block by block basis. Blocks free from error are written into addresses of the memory corresponding to the ID signals.
Only blocks of the reproduced video signals free from error are thus stored into the memory. Therefore, slow images reproduced and displayed on the basis of image signals read out from this memory become favorable.
By taking slow reproduction with a quarter speed as an example, such slow reproduction of a digital VTR using a frame memory will now be described concretely with reference to FIGS. 3 and 4. However, it is now assumed that each field of the image signals is so recorded on the magnetic tape as to be divided into two tracks. Therefore, image signals of half a field are recorded onto each track. Such a recording apparatus and method is referred to as a two-segment recording apparatus and method. The recording interval of each field is equivalent to an interval during which a rotary drum having a head mounted thereon makes two revolutions.
FIG. 3 shows a track pattern on a magnetic tape in a digital VTR and scanning trace of the reproduction head at the time of slow reproduction.
Assuming now in FIG. 3 that a certain frame of image signals (hereafter referred to as a frame of interest) is frame 0, its immediately preceding frame is referred to as the preceding frame and is represented as frame (-1), whereas its immediately succeeding frame is referred to as the succeeding frame and is represented as frame 1. Further, one frame comprises two fields. The preceding one of the two fields is referred to as field 0, and the succeeding one of the two fields is referred to as field 1.
Image signals of the preceding frame (-1) are recorded beforehand onto tracks T.sub.-1 (0. 1) to T.sub.-1 (1. 2). The former half of the field 0 of that frame is recorded beforehand onto the track T.sub.-1 (0. 1). Its latter half is recorded beforehand onto track T.sub.-1 (0. 2). The former half of field 1 of that frame is recorded beforehand onto track T.sub.-1 (1. 1), whereas its latter half is recorded beforehand onto track T.sub.-1 (1. 2). In the same way, image signals of frame 0 of interest are recorded beforehand onto four tracks T.sub.0 (0. 1) to T.sub.0 (1 2). Field 0 of the frame 0 is recorded beforehand onto two preceding tracks T.sub.0 (0. 1) and T.sub.0 (0. 2), and field 1 of the frame 0 is recorded beforehand onto two succeeding tracks T.sub.0 (1. 1) and T.sub.0 (1. 2). Further, the former half of field 0 of the succeeding frame 1 is recorded beforehand onto track T.sub.1 (0. 1).
In case of slow reproduction, the traveling speed of magnetic tape is different from that of normal recording on reproduction, as described above. Accordingly, the scanning trace of the reproduction head is inclined with respect to the recorded tracks. In some cases, therefore, the scanning trace extends over two adjacent tracks.
FIG. 3 shows scanning traces S.sub.1, S.sub.3, S.sub.5, S.sub.7 and S.sub.9 obtained in slow reproduction with a quarter speed when the reproduction head scans the tracks T.sub.0 (0. 1) to T.sub.0 (1. 2) having image signals of the frame 0 of interest recorded thereon. The scanning trace S.sub.1 is a scanning trace for the frame 0 of interest in the first revolution of the rotary drum. The scanning traces S.sub.3, S.sub.5 and S.sub.7 are scanning traces respectively in the third, fifth and seventh revolutions. Scanning traces in even-numbered revolutions are omitted for the sake of clarity.
It is now assumed that magnetic tape 16 travels at a speed equivalent to a quarter of that of normal reproduction in a direction indicated by an arrow A. In reproduction of image signals of this frame 0 of interest, the former half of the scanning trace S.sub.1 caused by the first revolution of the rotary drum lies upon the track T.sub.0 (0. 1) whereas the latter half of the scanning trace S.sub.1 lies upon the track T.sub.-1 (1. 2). As the second revolution, the third revolution and so on are made, the scanning trace moves successively to the upper right on the magnetic tape 16 as represented by S.sub.3, S.sub.5, S.sub.7, S.sub.9, --- as a result of traveling of the magnetic tape 16 in the direction indicated by the arrow A. In the first revolution of the rotary drum, therefore, the track T.sub.0 (0. 1) of the frame 0 of interest and a shaded region of the last track T.sub.-1 (1. 2) of the preceding frame (-1) are scanned for reproduction. As the second revolution and then the third revolution are made thereafter, a portion of the track T.sub.-1 (1. 2) reproduced and scanned successively decreases. In the fourth revolution, the track T.sub.-1 (1. 2) is not scanned for reproduction. Instead, the tracks T.sub.0 (0. 1) and T.sub.0 (0. 2) are scanned for reproduction. Further, commencing with the eighth revolution, the track T.sub.0 (1. 1) having the former half of the preceding field 0 in the frame 0 of interest recorded thereon is scanned for reproduction. Sixteen revolutions of the rotary drum results in reproduction of one frame.
In the former half of the scanning trace S.sub.1, therefore, image signals are reproduced from the track T.sub.0 (0. 1). In the latter half of the scanning trace S.sub.1, image signals are reproduced from the track T.sub.-1 (1. 2). As the scanning trace advances from S.sub.1 successively to S.sub.2, S.sub.3 and so on, however, the interval during which image signals are reproduced from the track T.sub.-1 (1. 2) becomes shorter. Commencing with scanning trace S.sub.4, the track T.sub.0 (0. 2) is scanned for reproduction instead. In the scanning traces S.sub.1 to S.sub.4, the whole of the track T.sub.0 (0. 1) is scanned for reproduction. In scanning traces S.sub.5 to S.sub.8, the whole of the track T.sub.0 (0. 2) is scanned for reproduction. Commencing with the scanning trace S.sub.8, the track T.sub.0 (1 1) is scanned for reproduction.
In case of slow reproduction with a quarter speed, image signals of one field are thus reproduced by performing scanning for reproduction eight times. Image signals of one frame are reproduced by performing scanning for reproduction sixteen times.
With regard to slow reproduction shown in FIG. 3, writing digital reproduced image signals into the frame memory will now be described by referring to FIGS. 4A to 4E.
FIGS. 4A to 4E show storage contents of the frame memory obtained when the rotary drum has made an odd number of revolutions. In order to indicate from which track shown in FIG. 3 the storage contents have been reproduced, each of the storage contents is provided with a call out given to each track of FIG. 3.
In the frame memory, storage areas of fields 0 and 1 are predetermined. In each field, blocks reproduced from the magnetic tape 16 are stored into their pertinent storage areas in order of reproduction. The storage areas respectively allocated in the frame memory to the fields 0 and 1 are referred to as field 0 storage area and field 1 storage area, respectively.
First of all, the scanning trace which immediately precedes the scanning trace S.sub.1 shown in FIG. 3 and which is not illustrated will now be described. Although not illustrated in FIGS. 4A to 4E, image signals reproduced from the track T.sub.-1 (0. 1) are stored into former half of the field 0 storage area of the frame memory, and image signals reproduced from the track T.sub.-1 (0. 2) are stored into latter half of the field 0 storage area. Image signals reproduced from the track T.sub.-1 (1. 1) are stored into former half of the field 1 storage area, and image signals reproduced from the track T.sub.-1 (1. 2) are stored into latter half of the field 1 storage area.
Under such a storage state in the frame memory, the first revolution of the rotary drum in the frame 0 of interest causing the scanning trace S.sub.1 is started. In the nearly former half of the track T.sub.0 (0 1), signal reproduction is thus performed. As shown in FIG. 4A, the resultant image signals are written into an area nearly equivalent to a quarter of the field 0 storage area in the frame memory beginning with the start address of the field 0 storage area. In nearly the latter half of the scanning trace S.sub.1, signals are reproduced from nearly the latter half (shaded portion) of the track T.sub.-1 (1. 2). As shown in FIG. 4A, the resultant reproduced video signals are written into the last portion of the field 1 storage area in the frame memory nearly occupying a quarter of the field 1 storage area. In this portion, image signals having the same contents as those of image signals already stored are written. As a result, its storage contents are not changed.
If with reference to FIG. 3 reproduction scanning along the scanning traces S.sub.2 (not illustrated) and S.sub.3 is performed by the second revolution and the third revolution of the rotary drum, the reproduction area of the track T.sub.0 (0. 1) expands. In the frame memory as well, the storage area of image signals reproduced from the track T.sub.0 (0. 1) expands in the former half of the field 0 storage area as shown in FIG. 4B. Consequently, the reproduction area of the track T.sub.-1 (1. 2) of the preceding frame (-1) decreases. The above described writing range in the field 1 storage area of the frame memory decreases.
In the same way, the track T.sub.0 (0. 2) also begins to be scanned for reproduction as a result of travel of the magnetic tape 16. In the fifth revolution of the rotary drum, image signals reproduced from the whole of the track T.sub.0 (0. 1) are stored into the former half of the field 0 storage area in the frame memory, and image signals reproduced from the former half of the track T.sub.0 (0. 2) are stored into an area occupying nearly a quarter of the latter half of the field storage area as shown in FIG. 4C. In the seventh revolution of the rotary drum, the reproduction area in the track T.sub.0 (0 2) expands, and the storage area of image signals reproduced from this track T.sub.0 (0. 2) thus expands in the field 0 storage area of the frame memory as shown in FIG. 4D. In the eighth and ninth revolutions of the rotary drum, image signals reproduced from the whole of the tracks T.sub.0 (0. 1) and T.sub.0 (0. 2) are written into the field 0 storage area of the frame memory, and image signals of the field 1 reproduced from the track T.sub.0 (1. 1) are written into the field 1 storage area of the frame memory as shown in FIG. 4E.
The write address of the reproduced image signals in the frame memory is specified on the basis of the ID signal added to each block.
In the frame memory, the writing operation heretofore described is performed. While this writing operation is performed, however, a reading operation is performed repeatedly.
Reading from the memory is performed alternately in the field 0 storage area and the field 1 storage area. Such slow reproduction image signals in which one frame comprises field 0 and field 1 are thus obtained.
As described above, the interval of two revolutions of the rotary drum is equivalent to one field interval. Therefore, the interval of four revolutions of the rotary drum is equivalent to one frame interval of image signals. In the frame memory in which a writing operation is conducted as shown in FIGS. 4A to 4E, therefore, the rewriting interval of reproduced image signals corresponding to one frame is equivalent to an interval during which the rotary drum makes sixteen revolutions, in case of slow reproduction with a quarter speed. During this interval, therefore, readout of the frame memory is repeated four (=16.div.4) times and hence image signals of four frames are obtained. That is to say, in an interval during which image signals of one frame are reproduced from the magnetic tape 16, image signals of four frames are obtained from the frame memory, resulting in slow reproduction with a quarter speed.
In some cases of the above described slow reproduction, however, image signals of another frame are simultaneously and mixedly reproduced. If a change is caused in such cases between frames of image signals representing a moving picture by a moving portion of the moving picture, blurring appears in the reproduced images.
It is now assumed that a certain frame begins to be read out in the storage state shown in FIG. 4A from the frame memory into which image signals have been written, as shown in FIGS. 4a to 4E. Image signals of one frame are then read out in the interval of four revolutions of the rotary drum. In the storage state shown in FIG. 4C, therefore, a subsequent frame begins to be read out. In the storage state shown in FIG. 4E, a further subsequent frame begins to be read out.
As for the interval of eight revolutions of the rotary drum shown in FIGS. 4A to 4D, the however, writing operation to the field 0 storage area of the frame memory is conducted in the interval of the first to third revolutions of the rotary drum. In image signals of the field 0 obtained by readout in this interval, field 0 of the frame 0 of interest reproduced from the magnetic tape 16 and field 0 of the preceding frame (-1) which precedes it by one frame are mixedly present. That is to say, if readout of a certain frame is started in the storage state of FIG. 4A and readout of a subsequent frame is started in the storage state of FIG. 4C, the following operation results. In the storage state of FIG. 4a, image signals reproduced from the track T.sub.-1 (0. 1) of the preceding frame (-1) shown in FIG. 3 are mixed into field 0 of image signals read out from the field 0 storage area of the frame memory. In the storage state of FIG. 4C, however, image signals reproduced from the track T.sub.0 (0. 1) of the frame 0 of interest are present on the area, in which image signals reproduced from this track T.sub.-1 (0. 1) of the frame memory were stored before, as a result of a rewriting operation.
Assuming now with regard to slow-reproduction images that images reproduced from the track T.sub.-1 (0. 1) are displayed in field 0 of a certain frame interval of image signals obtained from the frame memory, images reproduced from the track T.sub.0 (0. 1) are displayed in the same location as that of the foregoing description in the field 0 of a subsequent frame interval.
There is a time lag equivalent to one frame interval (which is 1/30 second in case of an apparatus and a method of NTSC type) between image contents recorded on the track T.sub.-1 (0. 1) and image contents recorded on the track T.sub.0 (0. 1). If a moving portion of image causing a lag is included in these image contents, therefore, discrepancy in position and size between images of successive frames displayed on an identical location of the slow-reproduction screen is caused, resulting in blurring. In general, favorable slow-reproduction images are obtained in case of slow reproduction with 1/n times speed (where n is an integer not less than 2) by repeating a frame having identical image contents n times. In the above described case, however, image contents change while the frame is being repeated n times. Discrepancy in position and size is thus caused in displayed images, resulting in blurring.